A nitride semiconductor has a high electric breakdown field and a high electron saturation velocity. Thus, the nitride semiconductor is expected to be a semiconductor material for semiconductor devices having a high breakdown voltage and a low on-state resistance. Many of the conventional semiconductor devices using the nitride related materials may have hetero junctions. The hetero junction is configured with two types of nitride semiconductors having different bandgap energy from each other and is able to generate a two-dimensional electron gas layer (2DEG layer) near the junction plane. The semiconductor devices having the hetero junction may achieve a low on-state resistance by passing an electric current through this 2DEG layer. This type of semiconductor devices is called a HEMT (High Electron Mobility Transistor).
This type of conventional semiconductor devices has a gate portion for switching the electric current. The gate portion has a hetero junction and a gate electrode that faces the hetero junction.
Generally, the hetero junction is formed at a c-plane of a semiconductor crystal. The c-plane has a strong internal electric field due to its spontaneous polarization and piezoelectric polarization. Therefore, electrons are able to stay near the hetero junction plane in high density. Thus, a 2DEG layer may be generated near the hetero junction plane with high density. This type of conventional semiconductor devices utilizes the 2DEG layer generated near the hetero junction plane as a channel, and as a result, achieves a low on-state resistance. However, this type of conventional semiconductor devices needs to apply a negative voltage to the gate electrode in order to eliminate the 2DEG layer. That is, this type of conventional semiconductor devices operates in a normally-on mode.
The document 1 listed below discloses a semiconductor device which operates in a normally-off mode. The gate portion of the semiconductor device described in the document 1 utilizes the hetero junction formed at an a-plane. The a-plane is a crystal plane that extends in a direction perpendicular to the c-plane. Thus, due to the spontaneous polarization and the piezoelectric polarization of the a-plane, the direction of the internal electric field therein is parallel to the hetero junction. Therefore, the a-plane has a characteristic feature of non-polarity. By utilizing this technique, the electron density near the hetero junction may be decreased and semiconductor devices may operate with the normally-off mode.    (Document 1): Masayuki Kuroda, Hidetoshi ISHIDA, Tetsuzo Ueda, and Tsuyoshi Tanaka, “Normally-off Operation of AlGaN/GaN Heterojunction Field Effect Transistors on Non-polar (11-20) plane”, IEICE (THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS) Technical Report, ED2005-205, MW2005-159(2006-1), pp. 35-39